Abstract
The underwater sonar system using an acoustic lens is receiving attention again because it does not need a complex electrical circuit and exhibits reduced volume and cost. However, in our former experiments, singlet spherical or aspherical lenses could not concentrate sound at a focal point completely because of aberrations. In this study, we tried to design a lens that can remove spherical and coma aberrations in a paraxial area using a ray tracing method, and two types of lens were designed. One is a biconcave acoustic lens with the refractive index of n=0.56, which corresponds to an acrylic resin, and the other is a biconvex acoustic lens with n=1.5, which corresponds to a silicon rubber. These lenses are evaluated by comparing of their sound pressure fields with those of a spherical lens and a plano-concave elliptic lens as calculated by a two-dimensional finite difference time domain (2D-FDTD) method. As a result, the biconcave aplanatic lens shows the best performance among these four lenses.